Figure 1. Cortical coherence with finger acceleration. A using sensorimotor EEG in 5 human subjects. B using LFP from 5 different M1 sites in a single monkey.

Figure 2. Average coherence spectra with finger acceleration. A using EEG in the human (n=8 subjects); B using LFP in monkey (n=10 sites). C coherence spectrum between finger acceleration and the average of 10 simultaneously recorded LFPs. D average of 15 spectra calculated as in (C) from different recording sessions.

During slow finger tracking movements, discontinuities can be seen in finger acceleration at ~10 Hz (Vallbo & Wessberg, 1993); whether the motor cortex (M1) contributes to this activity is uncertain. We recorded right finger acceleration and contralateral sensorimotor EEG (differential recording, electrodes 20 mm anterior and posterior and 30 mm lateral to vertex) from 8 normal human subjects performing slow index finger flexion-extension tracking movements. Coherence was calculated between EEG and acceleration during flexion. Significant coherence at 8-10 Hz was only found in 3/8 subjects (Fig. 1A). The mean coherence (Fig. 2A), averaged over all 8 subjects, showed a significant peak at 9.8 Hz, although this was weak (coherence = 0.018). We compared this result with data from a macaque monkey, trained to perform a similar slow finger flexion task for food reward. Following training, the animal was implanted (under general anaesthesia, 3.0-5.0% sevoflurane inhalation with 0.025 mg/kg/h alfentanil, and aseptic conditions) with a headpiece for head fixation, and a recording chamber. Daily experimental sessions made up to 10 simultaneous microelectrode penetrations into M1, recording local field potential (LFP; bandpass 1-100 Hz, referenced to the headpiece) whilst the animal performed the task. In contrast to the human data, LFP-accelerometer coherence often showed robust ~10 Hz peaks (Fig. 1B). The average coherence spectrum over 10 simultaneously recorded sites had a peak of 0.16 at 9.8 Hz (Fig. 2B). Out of 113 recording sites, 91 showed at least one significant bin in the 8-10 Hz band. The difference between LFP and EEG is intriguing, as they yield similar results for corticomuscular coherence at ~20 Hz during steady contractions. One possibility is that EEG sums spatially over a larger population than LFP, producing cancellation. We simulated this in the monkey by averaging together all LFPs simultaneously recorded in one session (maximum inter-electrode distance at the surface 2.1 mm), and then calculating coherence between averaged LFP and acceleration (Fig. 2C, for the same session as Figs 1B and 2B). The average of this spectrum over 15 sessions (3-10 LFPs/session, mean 7.5) showed considerable coherence at ~10 Hz (Fig. 2D). We conclude that M1 activity does contribute to 10 Hz discontinuities during slow finger movement.